ROCKS ALTERED BY GRANITE VEINS. 
5G1 
‘Fig. 614. 
Granite veins passing through hornblende slate, Carnsilver Cove, Cornwall. 
Granite, syenite, and those porphyries which have a gra- 
nitiform structure, in short all plutonic rocks, are frequently 
observed to contain metals, at or near their junction with 
stratified formations. On the other hand, the veins which 
traverse stratified rocks are, as a general law, more metallif¬ 
erous near such junctions than in other positions. Hence it 
has been inferred that these metals may have been spread in 
a gaseous form through the fused mass, and that the con¬ 
tact of another rock, in a dififerent state of temperature, or 
sometimes the existence of rents in other rocks in the vicini¬ 
ty, may have caused the sublimation of the metals.* 
Veins of pure quartz are often found in granite as in many 
stratified rocks, but they are not traceable, like veins of 
granite or trap, to large bodies of rock of similar composi¬ 
tion. They appear to Fig.eis. 
have been cracks, into 
which siliceous matter 
was infiltered. Such 
segregation, as it is 
called, can sometimes 
clearly be shown to 
have taken place long 
subsequently to the 
original consolidation 
of the containing: rock. 
Gneiss. 
Greenstone 
Dike. 
Gneiss. 
served in the gneiss of 
Tronstad Strand, near Drammen, in ^Norway, the annexed sec¬ 
tion on the beach. It appears that the alternating strata of 
whitish granitiform gneiss and black hornblende-schist were 
* Necker, Proceedings of Geol. Soc., No. 26, p. 892. 
24* 
